diamond microtip field emitters are used in diode and triode vacuum microelectronic devices, sensors and displays. diamond diode and triode devices having integral anode and grid structures can be fabricated. Ultra-sharp tips are formed on the emitters in a fabrication process in which diamond is deposited into mold cavities in a two-step deposition sequence. During deposition of the diamond, the carbon graphite content is carefully controlled to enhance emission performance. The tips or the emitters are treated by post-fabrication processes to further enhance performance.
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1. A diamond diode device comprising:
a. a cathode comprising at least one emitter structure formed monolithically on a diamond substrate, the emitter structure comprising solid diamond and including a diamond microtip emitting portion; and b. an anode positioned over and spaced apart from the diamond microtip emitting portion of the cathode; c. the cathode fabricated integral to the diamond microtip emitting portion to form a re-usable, integrated diamond diode device.
2. A diamond diode device comprising:
a. a cathode comprising at least one emitter structure formed on a diamond substrate, the emitter structure comprising solid polycrystalline diamond and including a diamond microtip emitting portion; b. an insulating layer formed on the emitter structure and diamond substrate but not covering the diamond microtip emitting portion; and c. a first conductive layer formed over the insulating layer but not covering the diamond microtip emitter portion, forming an anode.
3. The diamond diode device of
5. The diamond diode device of
6. The device of
7. The device of
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This application is a divisional of and claims benefit to U.S. patent application Ser. No. 08/883,417 filed Jun. 25, 1997 now U.S. Pat. No. 6,132,278, entitled "Mold Method for Forming Vacuum Field Emitters and Method for Forming Diamond Emitters."This application claims benefit of PROVISIONAL APPLICATION No. 60/020,428, filed Jun. 25, 1996.
The present invention relates to vacuum microelectronics and particularly to micro-patterned, microtip emitter structures made from diamond and similar materials, such as field emitter elements/arrays for use as vacuum diodes, triodes, sensors, displays, and other related applications.
The advance in integrated circuit fabrication and silicon micromachining technology has given an impetus to the development of vacuum microelectronic devices. Central to the field of vacuum microelectronics is the search for a high efficiency electron emission cathode. In recent years, many different materials, structures, and techniques have been investigated for fabrication of vacuum cold cathode devices. Examples of such materials, structures, and techniques are described in: H. F. Gray, Proc. 29th Intl. Field Emission Symp., p. 111, 1982; I. Brodie, IEEE Trans. on Electron Devices, 36, p. 2641, 1989; C. A Spindt, C. E. Holland, A. Rosengreen and I. Brodie, IEEE Trans. on Electron Devices, 38, p. 2355, 1991; E. A Adler, Z. Bardai, R. Forman, D. M. Goebel, R. T. Longo and M. Sokolich, IEEE Trans. on Electron Devices, 38, p. 2304, 1991; and M. Yuan, Q. Li, W. P. Kang, S. Tang and J. F. Xu, Journal of Vacuum Science Technology B, 12(2), p. 676-679, 1994. The most desirable properties for an electron emission cathode are low operating voltage, high emission current density and uniformity, and emission stability, longevity and reliability.
The unique material properties of diamond, including low electron affinity, wide band-gap, chemical stability, resistance to particle bombardment, hardness, and good thermal conductivity, are beneficial for vacuum microelectronics applications. However, due to the chemical inertness of diamond, the work reported in the prior art involves only planar diamond films, non-uniformly diamond coated silicon tips, or irregular ion-etched diamond conical structures. Control of the uniformity and microstructure of diamond film is essential for field emission device applications. Very high field emission current with diamond is achieved by proper design and configuration of a well structured diamond microtip emitter.
Although those skilled in the art have recognized that diamond has properties that make it potentially very useful as an emitter in microelectronic devices, that potential has remained unfulfilled up to now. Various emitter structures using diamond have been designed but their emission performance has been unsatisfactory. For example, many prior art diamond tipped emitter structures have been inefficient emitters or have produced emission currents that are unstable. To obtain high field emission efficiency in a solid state microstructure emitter, the tip of the emitter must be extremely sharp. In those few instances in the prior art where efficient and stable diamond tip emitters have been built, the fabrication techniques have been expensive and/or time consuming. Typically, the prior art to structures have been fabricated by a sputtering or deposition process that lays the diamond on a planar substrate. The resulting emitter structure must then go through extensive machining or other post-deposition shaping steps in an attempt to create a sharp tip that will perform adequately. In other prior art fabrication methods, additional steps must be taken to initiate diamond growth, such as by ion implantation of the substrate. For example, in U.S. Pat. No. 5,129,850, the inventors describe a method of fabricating an emitter having a diamond coating. Although the diamond coating may enhance the emission characteristics of the emitter (assuming that the device could actually be built as described), the device will not have the same desirable characteristics found in a solid, monolithic diamond emitter structure.
The fabrication and emission performance problems of the prior art have been overcome in the novel field emission devices and fabrication methods of this invention, using sharp tips of well patterned diamond microtip emitters (e.g., pyramid, knife edge, conical, volcanic cone, sharp pillar microstructures) for the development of vacuum field emitter element/arrays for vacuum microelectronics and sensor applications. The use of local electric field enhancement at sharp points, constructed by molding and micromachining techniques of diamond material as described here, utilizes plasma enhanced chemical vapor deposition (PECVD) to produce micron size or smaller structures, on a diamond film/field, with very sharp tip curvatures, such as less than 200 A. Several novel structures and devices are described, including a micro-patterned diamond emitter element/array, and related novel device structures in diode, triode, display, and sensor configurations.
To create the high performance diamond microtip structures of this invention, several novel fabrication steps are described, including deposition of diamond into cavities formed in a substrate mold, using a novel deposition process that preferably occurs in a sequence of smooth and standard deposition steps. In the smooth deposition step, small grain sizes are achieved at the tip of the emitter structure, with the standard deposition step producing larger grain sizes with an increased deposition rate.
The novel fabrication processes of this invention includes the ability to control the carbon graphite content of the diamond. This produces a diamond tip with an ideal or controllable balance of emission efficiency and durability. Emission performance of the structure is further enhanced by vacuum-thermal-electric treatment of the tips, hydrogen plasma tip sharpening, high temperature annealing, and application of thin metal coatings to the tips.
In another embodiment of the invention, diamond microtip emitters and emitter arrays are used as cathodes in novel diode and triode devices having integrated anode and/or grid structures.
High current emission from the patterned diamond microtip arrays was obtained at low electric fields. An emission current from the diamond microtips of 0.1 mA was observed for a field of <10 V/μm. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to silicon emitters. Moreover, field emission from patterned pyramidal polycrystalline diamond microtip emitter arrays, as fabricated by the inventors and described herein, is unique in that the applied field is found to be lower compared to that required for emission from Si, Ge, GaAs, and metal surfaces. The novel fabrication processes utilize selective deposition of diamond film in a cavity mold such as might be created in silicon, and subsequent creation of a free standing diamond substrate or plate with a diamond microtip emitter array. The processing techniques are compatible with IC and other micromachining fabrication technologies.
FIGS. 51(a), (b), and (c) are cutaway side views of a diamond triode device shown during and after fabrication as a fully integrated circuit in accordance with the present invention.
FIGS. 52(a), (b), and (c are cutaway side views of another embodiment of diamond triode device shown during and after fabrication as a fully integrated circuit in accordance with the present invention.
Fabrication of Diamond Microtips Using Substrate Molding
In the novel method of this invention, diamond (polycrystalline, crystalline, amorphous, monocrystalline, diamond-like carbon) field emitter structures and emitter arrays can be fabricated by PECVD or by other diamond deposition processes on a variety of molding substrates, such as semiconductor (Si, Ge, etc.), metal, or insulator (glass, silicon dioxide, etc.). A schematic description of such a process for fabricating an array of pyramidal diamond tipped microstructures using <100> type silicon as a molding substrate is shown in
Optionally in step (8), a second masking layer 14 of silicon dioxide is applied to the lower surface 13 of the molding substrate 10, with all of masking layer 14 and the molding substrate 10 etched away except around the periphery of the substrate 10. This leaves, in step (9), an array of diamond emitters 25 arranged monolithically on an integral diamond substrate 30, with each emitter 25 having a pyramidal shape and a sharply pointed microtip 26 that substantially conforms to the inverted pyramidal shape of its corresponding cavity 11.
In accordance with another novel feature of this method, the diamond is deposited in two distinct sequential processing steps in order to initiate the diamond growth into the cavities 11 and the subsequent deposition of the diamond substrate 30. The primary purpose in using two sequential to processing steps in depositing the diamond is to insure that the tip portion 26 of each emitter 25 is as sharp as possible with improved emission efficiency. To accomplish this, the first or "smooth" deposition step is designed to deposit diamond at and near the tip 26 having a small grain size, preferably less than 2000 Angstroms in breadth. The second or "standard" step is used to complete the deposition of diamond in the cavities and/or across the top surface 12 of the molding substrate 10 at an increased deposition rate to form the diamond substrate 30. Consequently, the grain size of the diamond deposited in the standard deposition step will be larger, but this will not substantially impair the performance of the emitter 25.
One advantage of the method of this invention is that a diamond emitter or emitter array can be fabricated monolithically with an integral diamond substrate. However, the method can also be used to fill the cavities 11 only, with an emitter substrate layer 30 (of diamond or of a different material) created and bonded to the back of the emitters 25 in a separate step. Also, although a two step (smooth then standard) deposition of diamond is preferred, in some applications (such as very small emitters and cavities) the smooth deposition step could be used alone or a third step could be added for purposes of controlling another parameter of the emitter, such as the doping level or carbon graphite content of the diamond.
Achieving a small diamond grain size in the smooth deposition step requires careful control of the deposition process parameters, including deposition energy, methane and hydrogen gas concentration and ratio, chamber pressure, and substrate temperature. A person skilled in diamond plasma deposition techniques can adjust the process parameters of the equipment being used to achieve the small grain sizes needed to carry out the smooth deposition step of this invention. For deposition of diamond using PECVD, examples of fabrication processing parameters are described below that can be used for making different types of diamond microtip emitters.
For a regular diamond microtip emitter, the process parameters for the smooth deposition step are:
Heater (substrate) temperature = 860°C C. | |
Chamber pressure = 12 torr | |
Hydrogen gas flow = 396 sccm | |
Methane gas flow 4 sccm | |
Microwave power = 650 watts | |
Deposition time = 7 hours | |
The process parameters for the standard deposition step are:
Heater (substrate) temperature = 850°C C. | |
Chamber pressure = 40 torr | |
Hydrogen gas flow = 500 sccm | |
Methane gas flow = 5 sccm | |
Microwave power = 1500 watts | |
Deposition time = 13 hours | |
For a highly graphitic diamond microtip, the following process parameters should be used for a PECVD smooth deposition step:
Heater temperature = 800°C C. | |
Chamber pressure = 12 torr | |
Hydrogen gas flow = 135 sccm | |
Methane gas flow = 15 sccm | |
Microwave power = 550 watts | |
Deposition time = 40 hours | |
For a diamond microtip emitter of low graphitic content, the following process parameters should be used for the smooth deposition step:
Heater temperature = 875°C C. | |
Chamber pressure = 10 torr | |
Hydrogen gas flow = 396 sccm | |
Methane gas flow = 4 sccm | |
Microwave power = 650 watts | |
Deposition time = 1 hour | |
The process parameters for the standard deposition step then are:
Heater temperature = 875°C C. | |
Chamber pressure = 60 torr | |
Hydrogen gas flow = 500 sccm | |
Methane gas flow = 5 sccm | |
Microwave power = 1500 watts | |
Deposition time = 19 hours | |
Generally speaking, the higher the carbon graphite content in the diamond in the emitter, the higher the emission efficiency. As the sp2 (carbon graphite) content increases, the voltage needed to "turn on" the electric field from the emitter can be reduced from 80 V/μm to 15 V/μm. However, at very high levels of carbon graphite content, the durability of the emitter and tip can be impaired. Therefore, in accordance with another objective and novel feature of this invention, the carbon graphite content of the diamond is controlled during deposition to achieve an optimum combination of field emission efficiency and tip durability.
Control of the sp2 content in diamond film deposition by PECVD requires varying the process parameters and/or a combination of the sequence of process parameters. The most critical process parameter that controls sp2 content formation in diamond film is the ratio of methane to hydrogen gas concentration. The higher the methane concentration, the higher the sp2 content in the diamond. A second critical process parameter that controls sp2 content formation in diamond film is the plasma deposition energy level which, in the case of PECVD, is determined by the microwave power level. The microwave power determines the energy of the hydrogen plasma. The higher the energy level of the hydrogen plasma, the more sp2 content etching occurs. Therefore, low microwave power for all high sp2 content steps and high microwave power for all pure diamond steps should be used.
The substrate temperature also has an effect on sp2 content formation in diamond film. A lower substrate temperature should be used for all high sp2 content steps in order to prevent a secondary effect from the hydrogen plasma etching. Finally, the chamber pressure has a secondary effect on sp2 content formation in diamond film. The higher the chamber pressure, the more sp2 content etching by the hydrogen plasma is obtained because the hydrogen plasma is condensed by pressure so that its effective power is increased. Therefore, low pressure should be used for all high sp2 content steps and high pressure is used for all low sp2 content diamond steps.
Examples of process parameters for PECVD diamond deposition during fabrication of diamond (doped and undoped) microtip emitters with controlled carbon graphite content are described below. In some cases, a third deposition step is used to optimize the structure.
TABLE 1 | ||
Process parameters for undoped low sp2 content diamond film. | ||
Smooth Step | Standard Step | |
Number of steps = 2 | fine diamond | low sp2 content |
Substrate Temperature (C) | 875 | 875 |
Chamber Pressure (Torr) | 10 | 60 |
Hydrogen gas flow rate (sccm) | 396 | 500 |
Methane gas flow rate (sccm) | 4 | 5 |
Microwave power level (W) | 650 | 550 |
Time (hours) | 1 | 40 |
TABLE 2 | |||
Processing parameters for undoped moderate sp2 content diamond film. | |||
Step 1 | Step 2 | Step 3 | |
high sp2 | fine | standard | |
Number of steps = 3 | diamond | diamond | diamond |
Substrate Temperature (C) | 800 | 860 | 850 |
Chamber Pressure (Torr) | 10 | 11.8 | 40 |
Hydrogen gas flow rate (sccm) | 138 | 396 | 500 |
Methane gas flow rate (sccm) | 15 | 4 | 5 |
Microwave power level (W) | 550 | 650 | 1500 |
Time (hours) | ¾ | 7 | 13 |
TABLE 3 | |
Processing parameters for undoped high sp2 content diamond film. | |
Number of steps = 1 | Step 1 high sp2 content diamond |
Substrate Temperature (C) | 800 |
Chamber Pressure (Torr) | 11.8 |
Hydrogen gas flow rate (sccm) | 135 |
Methane gas flow rate (sccm) | 25 |
Microwave power level (W) | 550 |
Time (hours) | 45 |
TABLE 4 | |||
Process parameters for p-type moderate sp2 content diamond film. | |||
Step 1 | Step 2 | Step 3 | |
high sp2 | fine | standard | |
Number of steps = 3 | diamond | diamond | diamond |
Substrate Temperature (C) | 875 | 875 | 575 |
Chamber Pressure (Torr) | 10 | 11.8 | 40 |
Hydrogen gas flow rate (sccm) | 138 | 396 | 500 |
Methane gas flow rate (sccm) | 15 | 4 | 5 |
Microwave power level (W) | 550 | 650 | 1500 |
Time (hours) | ¾ | 7 | 13 |
TABLE 5 | |
Process parameters for p-type high sp2 content diamond film. | |
Number of steps = 1 | Step 1 high sp2 content diamond |
Substrate Temperature (C) | 900 |
Chamber Pressure (Torr) | 11.8 |
Hydrogen gas flow rate (sccm) | 135 |
Methane gas flow rate (sccm) | 25 |
Microwave power level (W) | 550 |
Time (hours) | 45 |
In the examples described above, the p-type diamond film was fabricated via a conventional in situ boron solid source doping method and n-type diamond film was fabricated using conventional in situ gas phase doping. Selective etching of the silicon molding substrate 10 (
Of course, useful diamond microtip emitters can be fabricated in other than pyramidal shapes. For example, some applications may benefit from using an emitter 25 in the shape of a high aspect ratio pillar, as shown in
The fabrication methods of this invention can also be employed so that the molding substrate is re-usable. This technique is illustrated in FIG. 9. Before the diamond is deposited, a release layer 16 of a material such as silicon dioxide is deposited into the cavities 11 and across the top surface 12 of a silicon molding substrate 10. After the diamond (emitters 25 and substrate 30) is deposited over the release layer 16, the diamond microtip emitters 25 and substrate 30 can be separated from the release layer 16 by thermal differential or controlled reduction, leaving the molding substrate intact. The release layer 16 is then etched away.
A scanning electron microscopy (SEM) photograph of a pyramidal diamond microtip emitter 25 on a free standing diamond substrate 30, fabricated using the method of this invention, is shown in FIG. 10. The emitter 25 at its base is approximately 3 μm×3 μm, with a radius at the tip 26 of less than 200 A.
Treatment of Diamond Microtip Emitters to Enhance Emission Performance
The electron field emission behavior of the emitter 25 can be improved by subjecting the post fabricated diamond microtip emitter 25 to hydrogen plasma treatment. Depending on the hydrogen plasma power and treatment time, the radius of the diamond tip 26 can be sharpened tothe nanometer range. Typical post fabrication hydrogen plasma treatment parameters are:
Heater (substrate) temperature = 850°C | |
Chamber pressure = 40 torr | |
Hydrogen gas flow = 500 sccm | |
Microwave power = 1500 watts | |
Treatment time = several minutes to the range of an hour | |
High temperature annealing processes, such as high temperature activation or initiation, can also be applied to improve the diamond tip field emission behavior. For example, activation of a gated emitter device can be conducted under the following thermal and electric conditions:
Baking the device at 250°C in vacuum and applying a gate voltage to keep the device operation at a low emission state for at least several hours. Attention must be paid in this process to avoid the rise of emission current to a value more than 2 μa. The device performance can be greatly improved after this treatment. The onset gate voltage is reduced and the emission is stabilized.
As discussed above, this invention demonstrates that the field emission characteristics of the diamond can be significantly improved by increasing the sp2 content of the diamond microtip emitter. To further enhance this characteristic, a vacuum-thermal-electric (VTE) treatment step can be performed on the fabricated emitters. As the sp2 content of the diamond increases, the turn-on electric field is reduced from 40 V/μm to 4 V/μm. That is, for low sp2 content diamond tips, the turn-on electric field is reduced from 80 V/μm to 40 V/μm after VTE treatment and for high sp2 content diamond tips, the turn-on electric field is reduced from 15 V/μm to 4 V/μm after VTE treatment. Therefore, the turn-on electric field can be reduced more than 50% by increasing the sp2 content and VTE treatment. The relationship between emitter efficiency and sp2 content of the diamond is shown in FIG. 38.
The novel vacuum-thermal-electric treatment of the diamond microtip emitter of this invention will improve the performance and stability of the diamond tip. A typical VTE treatment of the emitter can be carried out as follows:
At room temperature and in the vacuum environment of 10-6 Torr, the fabricated diamond microtip emitters are subjected to voltages that are gradually increased from zero until a significant emission current is detected. At the beginning stage, the emission is usually unstable because there is contamination on the tip surface. Vacuum-thermal-electric (VTE) treatment is conducted on the emitters by heating the emitter device slowly to approximately 150 C. in the same vacuum environment of 10-6 Torr or better. The device is maintained at 150 C. for several hours while the emission current is kept below 2 μA by adjusting the applied voltage. The VTE treatment is terminated when a stable current is obtained for a considerable period of time, usually 1 hour. The devices are then cooled down slowly to room temperature. The emission currents after VTE treatment were confirmed to be significantly improved in terms of performance and stability.
Field emission of the diamond microtip emitter can also be improved by surface treatment of the emitters with a thin-film metal coating. Certain types of metal coatings (gold, for example), in conjunction with control of the sp2 content of diamond emitter, can enhance the emission by: (i) lowering the work function due to the sp2 defect induced band and gold induced negative electron affinity on the diamond tip surface; and (ii) the increase in field enhancement factor due to the enhanced electric field via the metal-insulator-metal microstructure and field forming process in the tip region.
To apply a performance enhancing gold coating to the emitters in accordance with this invention, the following steps can be used.
After the polycrystalline diamond film deposition, the silicon molding substrate 10 was etched away with a mixture of HF:HNO3 (2:1) solution. The emitters 25 were then cleaned with acetone and methanol. The emitter samples were cleaned with acetone and dehydrated at 300 C. for 5 minutes in order to make the surface clean. A thin layer of gold with a nominal thickness of 200-250 Angstroms was sputtered on the surface of the emitter tip 26. The emitters 25 were then annealed at 900 C. for 10-15 minutes.
The change in emitter performance at different levels of carbon graphite content is shown in
The geometry of the tip portion 26 of the emitter 25 plays an important role in field emission. The use of oxygen or hydrogen plasma for micro etching can produce an ultra-sharp tip for improved field emission. For example,
Finally, thermal oxidation of the cavities in a silicon or other molding substrate, before diamond deposition, can improve the geometry of the tip portion 26 of the emitter 25. Due to the preferential oxidation of silicon, the tip of the cavity in silicon is ultra-thin thereby producing an ultra-sharp diamond tip.
Diamond Microtip Emitters for Vacuum Diode and Triode Devices
The diamond microtip emitter structures as fabricated and described above can be employed in a variety of novel diamond vacuum diode and triode configurations. In such devices, operation in the field emission process is controlled by Fowler-Nordheim tunneling, as described in C. A. Spindt, et al., "Research into Micron-size Emission Tubes", IEEE Conf. on Tube Techniques, 1966.
The emission current density is given by:
where K1 and K2 are constants, and Φ is the work function of the emitting surface.
The electric field E is defined as:
where V is the anode-cathode voltage, β is the field enhancement factor controlled by the cathode radius of curvature at the point of emission, and d is the spacing between the cathode (emitter) tip and the anode.
The field at the apex of the tip is inversely proportional to the tip radius. The sharp needle tips of the diamond-microtip pyramidal emitter structures are fabricated to enhance the electric field at the apex and promote high emission current.
The Fowler-Nordheim (F-N) field emission behavior of the diamond microtip emitters 25 of
A comparison of the emission characteristics between diamond emitters and silicon tips of the same type of array structures is shown in FIG. 20. It can be seen that electron field emission for the diamond microtip emitters, in forward bias, exhibits significant enhancement both in total emission current and stability compared to the silicon emitters. The applied electric field for emission is at least 10 to 100 times lower than that required for emission from the best reported values for Si, Ge, GaAs, and metal surfaces.
Another embodiment of a diode device fabricated from the novel diamond microtip emitters of this invention is shown in FIG. 13. The emitter 25 and substrate 30 are made of doped diamond, forming the cathode. The anode 35 is also doped diamond, separated by an insulating layer 40 of intrinsic diamond.
In
The diode device of
In the device of
The diamond triode device of
The diamond diode and triode devices of this invention can be fabricated using a process substantially as follows:
The fabrication process is started by growing a 1-1.5 μm oxide masking layer on a p-type <100> silicon wafer as a molding substrate. Conventional photolithography is then used to define the mask layer array. Elements of the array were defined by square pattern oxide windows in the masking layer. To form the inverted pyramidal emitter structures, cavities were anisotropically etched into the molding substrate with an etch-stopped process using potassium hydroxide:normal propanol:deionized water solution. The substrate was then cleaned with acetone and methanol.
CVD diamond was deposited into the cavities and on the surface of the molding substrate by PECVD. The molding substrate material was then etched away.
In accordance with another novel feature of this invention, additional fabrication steps were performed to construct the self-aligning gated emitter device of FIG. 44. The gated diamond emitter device of
In step (a), the array of diamond emitters and integral substrate is attached to a conductive layer, which is in turn bonded to a glass layer. The silicon molding substrate is then etched away in step (b). In step (c), an insulating gate dielectric layer (two micron thick silicon dioxide, for example) is applied over the emitter array and substrate. A conducting gate layer (of one micron thick aluminum or other etchable metal for example) is then applied over the insulating layer in step (d), followed by a layer of photoresist (e.g., an organic polymer) in step (e). The photoresist is applied so that the photoresist layer over the tips of the emitters is not as thick as it is over the regions adjacent to the emitters.
In step (f), the photoresist is partially removed using an ion milling or to other conventional technique that will remove the photoresist over the emitter tips first. Any technique that either uniformly etches the photoresist or that tries to make the photoresist layer flat will work. This produces an intermediate structure as shown in step (g). The metal (aluminum) layer is then exposed to the etch to remove the aluminum over the emitter tip and to partially undercut the photoresist in that region, as shown in step (h). Finally, in step (i), the balance of the photoresist is removed, leaving a device as in
To complete the gated vacuum diode or triode structure, a cap can be applied over the device, as shown in
The emission characteristics of the gated diamond emitters were tested in a vacuum environment of 10-6 torr.
Another novel aspect of this invention uses silicon integrated circuit technology to produce integrated diamond vacuum microelectronics with one mask and high yield capability. An embodiment of a planar device fabricated in this fashion is shown in
Another novel method of fabricating the diamond microtip diode and triode devices of this invention is illustrated in
Cavities are etched into a silicon wafer substrate 10 as described above. A dielectric layer 31 of silicon dioxide, for example, is deposited or grown on the substrate 10. The diamond is then deposited into the cavities and over the top surface of the substrate 10 to form one or more diamond at microtip emitters 25 and an integral diamond substrate 30, as shown on FIG. 50(a). The opposite surface of the substrate 30 is then bonded to a supporting substrate 34 (glass, for example) through a conducting layer 33, as seen on FIG. 51(b). Optionally, the conducting layer 33 can be omitted if the substrate 30 is sufficiently conductive or if some other conventional means of electrically contacting the emitter 25 is provided. A portion of the silicon molding substrate 10 is then removed by etching or lapping and polishing, back to the line 19, until the dielectric (silicon dioxide) layer 31 is exposed at segments 18. The remaining molding substrate 10 is then mechanically and chemically polished. As seen in FIG. 51(b), the device is then turned over, and the remaining "islands" of molding substrate 10 can be utilized as the anode of a diode or as the grid of a triode. As seen in FIG. 51(c), the dielectric layer 31 proximate the emitter tips 26 (segments 18) is etched away to expose the tips 26. To make a triode as seen in FIG. 51(c), an additional oxide layer 37 is deposited on the upper surface of the molding substrate 10, followed by deposition and etching of a metal layer to form anode 35. The remaining "islands" of the molding substrate 10 become the grid 45 of the triode device.
FIGS. 52(a)-(c) illustrate another method of fabricating a diamond vacuum diode or triode device. A masking layer (e.g., an oxide) is applied to a molding substrate 10 made of silicon, for example The mask is used to define a diffusion region within the substrate so that a buried layer 17 of highly doped silicon (FIG. 52(a)) is created in the substrate 10, to function as an integral anode or grid. After diffusion of the buried layer 17, a conventional epitaxy step is performed to grow a single crystal silicon epi layer 21 on a top portion of the molding substrate 10, such that epi layer 21 it becomes an extension of the molding substrate 10. Fabrication then proceeds as described for the device of FIGS. 51(a)-(c), yielding a diode device as seen in FIG. 52(b), with the buried layer 17 functioning as the anode, or a triode device with the buried layer 17 as the grid in the device of FIG. 52(c).
In the illustrations of the diamond diode and triode structures discussed above, the gate, grid, or anode elements are typically shown as ring structures that would encircle the emitters if seen in plan view. However, the same devices can also be fabricated in different architectures, such as using a horseshoe shape around the emitters or having a row of emitters between rows of the other elements on one or both sides.
Diamond Microtip Emitter Based Physical Sensors
The high electron emission efficiency of a diamond based cathode, the low operating voltage, high emission current density and uniformity, and emission stability, are desirable for microsensor applications in both terrestrial and space environments. Advanced novel device structures in vacuum microelectronics using diamond field emitters as electron sources can be utilized in microsensors such as pressure sensors, accelerometers, tactile sensors, and so on.
In this invention, a novel field emission device using an array of well patterned microtip emitters of diamond is employed as a microsensor. Diamond based physical sensor operation in the field emission process is controlled by the Fowler-Nordheim tunneling process. The conduction current density is given by equations 1 and 2 above. Based on the principle described above, the field at the apex of the tip is inversely proportional to the tip radius. Sharp needle tips of diamond-based pyramidal emitter structures are fabricated to enhance high electric field at the apex which yield high emission current. For physical sensor applications, the spacing d between the cathode tip and the anode must be deflection sensitive. This deflection sensitive parameter d can be realized by using a flexible membrane or cantilever beam as the anode or cathode, where a change in force, pressure, inertia, or acceleration can induce a change in the d value and hence in the emission current or device voltage.
In conjunction with selective growth of diamond microtips and IC compatible technology, useful diamond emitters and emitter arrays for sensor applications can be achieved. A schematic diagram of a typical pressure sensor structure is shown in
The pressure sensor of
Another embodiment of a diamond microtip diode structure configured as an accelerometer is shown in FIG. 30. In this embodiment, the anode 35 (diamond, for example) is flexible, with a mass 42 added to enhance movement of the anode 35 toward or away from the emitter 25 in response to a change in velocity.
Diamond Microtip Emitter Based Chemical Sensors
Novel diamond microtip emitter based chemical sensors which operate in the field emission process can be fabricated by integrating a thin coating of chemical sensitive material 43 on the surface of a pyramidal diamond microtip emitter 25, as shown in the diamond diode chemical sensor of FIG. 32. The chemically sensitive material 43 can be a catalytic metal such as Pt or Pd, and can be thermally evaporated, sputtered or otherwise deposited on the emitter 25. As a target gas or other chemical enters the chamber 38 through windows 44 in anode 35, the emitter will adsorb some of it. A change in work function of the catalytic metal upon adsorption of a chemical species by the catalytic metal coated on the diamond tip will modulate the election emission behavior and hence allow detection of the chemical.
The emission current density of a diamond microtip chemical sensor coated with a catalytic metal is given by:
Jb=K1(E2/Φb)exp ([-(K2Φb3/2)/E] in chemical species b (detecting) environment
where K1 and K2 are constants, and Φ is the work function of the emitting surface (in this case the catalytic metal coated on the diamond tip). Therefore, a change in chemical environment will alter the electron emission behavior of the device for chemical sensing.
Another embodiment of a chemical sensor in accordance with the present invention is shown in FIG. 33. The chemically sensitive layer 43 (a catalytic metal, for example) is applied over the diamond microtip emitter 25, to followed by an insulating layer 40 and a metal anode 35.
Novel Diamond Microtip Flat Panel Displays
Thus, although there have been described particular embodiments of the present invention of new and useful microtip vacuum field emitter structures, arrays, and devices, and method of fabrication, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims. Further, although numerous references and examples have been provided to specific materials being used for the various layers of structures and devices, it will be recognized by those skilled in the art that other materials having similar structural and/or electrical properties can be substituted without departing from the scope of this invention as claimed.
Kerns, Jr., David V., Kang, Weng Poo, Davidson, Jimmy Lee
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